Cisco LAN2LAN Software

Document ID: 14994

Contents

Introduction
Prerequisites
      Requirements
      Conventions
Initialization Procedure
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Introduction

LAN²LAN Technical Bulletin #7

Follow these guidelines when you initialize the LAN²LAN Lines.

Prerequisites

Requirements

There are no specific requirements for this document.

Conventions

Refer to Cisco Technical Tips Conventions for more information on document conventions.

Initialization Procedure

When a line has retried SABMs N2 times (currently N2 = 2), these actions are taken:

1. The output queue is purged; this includes any packets in the ring and any packets queued in the FIFO.

2. The line is declared down to the driver in respect of SCBs from the Router.

3. The state of the line is, "Remote not responding," to report purposes.

4. A line quality check is performed.

5. If the remote end is not observed to respond to the quality check, the state stays, "Remote not responding."

6. If the quality check fails, the state of the line is, "Failing quality check."

7. If the quality check message contains an unmodified loop back detection byte, the state of the line is "Looped back."

8. After a router receives the fourth quality check message or echo correctly, it measures the speed of the line when it times the transmission of a test frame.

9. After it completes the speed check, the router attempts to bring up the HDLC protocol and sends SABMs. When the SABMs succeed, the line enters the "In service" state.

10. If the SABMs do not succeed after two retries, the fall-back is "Remote not responding," and the procedure starts over.

A quality check consists of sending TEST commands, which are echoed by the partner router and returned as TEST responses. Only one router runs the sequence since it tests the line in both directions. Both routers send TEST commands, and the one with the highest node address defers to the other one. The deferring router assumes the router with line quality check responsibility will complete the test and send an SABM. If a role reversal occurs (the node address received in a TEST command becomes higher), the router which deferred starts to send TEST commands.

TEST commands are sent after each TEST response is received or a frame timeout occurs. Each TEST command contains the six-byte local node ID, a loop back detector byte, and a reasonably long data pattern. The data pattern is varied in four different ways: 1) all zeros, 2) all ones, 3) alternating ones and zeros, and 4) an ascending count in successive bytes. A router that echoes a TEST command XORs the loopback detection byte with itself.

A router that receives a TEST frame differentiates commands from responses when it checks the node router ID field and checks responses for loop back to make sure that the loop back detection byte is reversed. If loop back is detected, this status is added to the reported modem status. The test is restarted every time a bad CRC, loop back, or frame timeout occurs. If all four test frames are sent and correctly echoed, the router starts to send SABMs. The remote router will UA to an SABM.

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